1. Field of the Invention
This invention relates to integrated circuit manufacturing and more particularly to a method of determining the presence of electrically charged ionic contaminants within a PECVD silicon dioxide (oxide) layer deposited on a semiconductor substrate.
2. Description of the Relevant Art
Plasma-enhanced chemical vapor deposition (PECVD) techniques are often used to deposit layers of electrically insulating materials between layers of conducting traces called interconnects. Interconnects are patterned from layers of electrically conductive materials (e.g., aluminum, doped polysilicon, etc.) formed on the surface of a silicon substrate. Substantially coplanar interconnects formed on one layer must be electrically separated from interconnects of other layers. Electrical separation is achieved by placing an electrically insulating material (i.e., interlevel dielectric layer) between the vertically spaced interconnect layers. Several layers of interconnects may therefore be separated by interlevel dielectric layers. Interconnects at different elevational levels may be electrically connected by contacts formed in holes etched through the interlevel dielectric layers. Multiple layers (or levels) of interconnects allow a substantial increase in the density of devices formed on a semiconductor substrate. Common interlevel dielectric materials include silicon dioxide (oxide) and silicon nitride.
During PECVD deposition, a glow discharge (i.e., a plasma) is formed when radio frequency (RF) power is applied between two electrodes in a reaction chamber. Reactant gases contained in the reaction chamber produce chemically reactive species (atoms, ions, and radicals). These reactive species diffuse to an exposed surface of a target material, and are adsorbed on that surface. Chemical reactions occur on the exposed surface, resulting in the formation of a layer of desired material on the exposed surface of the target material.
The presence of electrically charged ions within dielectric layers of metal oxide semiconductor (MOS) devices are known to cause device reliability problems. Ionized alkali metal atoms (e.g., Na.sup.+ and K.sup.+) are very mobile in oxide layers, and move through gate oxides of MOS devices under the influence of the electric fields generated between gate electrodes and substrates during device operation. Over time, mobile ions in gate oxides tend to drift to the interface between the gate oxide and the underlying substrate. Resulting changes in MOS device threshold voltage levels may become large enough to cause circuits which incorporate these MOS devices to fail to meet electrical or performance requirements.
As device dimensions shrink, the influence of ionic charges in PECVD interlevel dielectric layers is being studied. Of particular interest is the influence of high concentrations of heavy metal ions (e.g., Fe.sup.++ and Cu.sup.++) found in PECVD dielectrics on the electrical properties of these dielectrics. While many techniques for determining the presence and concentrations of metal ions in dielectric layers currently exist, all are destructive in nature and require substantial sample preparation. These techniques include capacitance-voltage (C-V) measurements, secondary ion mass spectroscopy (SIMS), Auger emission spectroscopy (AES), and X-ray emission spectroscopy (XES).
Common (C/V) methods involve the formation of one or more MOS capacitors at test sites or on test wafers. When MOS gate structures are formed during a wafer fabrication process, MOS capacitors are also formed in test areas. MOS capacitors consist of a gate electrode formed over a gate oxide layer, the gate oxide layer being formed on a surface of a semiconductor substrate. Test areas may be on product wafers or on test wafers.
A first step in a typical C-V method involves measuring the capacitance of an MOS capacitor using high frequency alternating current (AC) stimulus while a direct current (DC) bias voltage applied between the gate electrode and the substrate is varied. The range of the applied DC bias voltage is sufficient to first deplete the surface of the substrate directly under the gate electrode of charge carriers, then attract oppositely-charged species to (i.e., invert) the surface of the substrate directly under the gate electrode. Measured capacitance values are then plotted versus corresponding values of applied DC bias, forming a first C-V curve. The MOS capacitor is then heated to about 300.degree. C. for about 30 minutes while being subjected to a high positive DC bias applied between the gate electrode and the substrate (i.e., bias-temperature treatment). The bias-temperature treatment causes mobile ionic charges to drift to the interface between the gate oxide and the underlying substrate. The MOS capacitor is then cooled to room temperature with the positive bias still applied. The capacitance of the MOS capacitor is again measured using high frequency AC stimulus while a DC bias voltage applied between the gate electrode and the substrate is varied. The measured capacitance values are plotted versus corresponding values of applied DC bias as a second C-V curve on the same graph as the first C-V curve. Any horizontal shift between the first and second C-V curves is directly proportional to number of mobile ionic charges in the oxide.
C-V methods depend on the ability to cause highly mobile ions in a dielectric layer to move within the dielectric, creating a change in bias voltage levels required to first deplete and then invert the surface of the substrate directly under the gate electrode. Heavy metal ions commonly found in PECVD dielectric layers such as Fe.sup.++ and Cu.sup.++ are not highly mobile. C-V techniques would thus be ineffective in detecting the presence of heavy metal ions in relatively thick PECVD dielectric layers.
Quantitative analytical methods such as secondary ion mass spectroscopy (SIMS), Auger emission spectroscopy (AES), and X-ray emission spectroscopy (XES) are surface analysis techniques. The depth profiling required for determining the concentrations of heavy metal ions in a sample thus requires repetition of the steps of surface analysis followed by removal of a thin layer of material at the upper surface of the sample. These techniques are very time consuming and are destructive in nature, requiring expendable samples. Such tests cannot be routinely performed economically, nor can they be performed directly on manufactured products.
A new non-contact technique for detecting the presence of mobile ions in an oxide layer involves a bias-temperature treatment similar to C-V methods. In this case, however, an electric field is created by depositing corona charge (positive or negative) on the upper surface of an oxide layer. A subsequent heating step induces mobile ions drift as in C-V methods. Measurements of contact potential difference between the semiconductor substrate and the material of a probe electrode positioned immediately above the oxide layer are made using a vibrating Kelvin probe. A change of contact potential difference before and after mobile ion drift indicates the presence of mobile ions in the oxide layer. To ensure any injection of charge into the oxide layer from the substrate or the extraction of charge from the oxide layer into the substrate during testing does not go undetected, measurements of the surface barrier potential of the semiconductor substrate before and after mobile ion drift are made using a surface photovoltage (SPV) technique. See, P. Edelman, et al., "New Approach to Measuring Oxide Charge and Mobile Ion Concentration," SPIE, March, 1994 (herein incorporated by reference).
Although the new contact potential difference technique described above is non-contact, the new technique is not suitable for detecting the presence of heavy metal ions in PECVD oxide layers of manufactured products. As with common C-V methods, the new contact potential difference method depends on the ability to cause highly mobile ions in a dielectric layer to move under the influence of an electric field. Since heavy metal ions commonly found in PECVD dielectric layers such as Fe.sup.++ and Cu.sup.++ are not highly mobile, the new contact potential difference technique would be ineffective in detecting the presence of heavy metal ions in relatively thick PECVD dielectric layers. In addition, manufactured products may be damaged or contaminated during the deposition of corona charge and the subsequent heating step.
It would thus be desirable to have a method of detecting the presence of heavy metal ionic contaminants in a PECVD oxide layer of a manufactured product.